1. Field of the Invention
The present general inventive concept relates to a multi-channel digital amplifier system, and more particularly, to a multi-channel digital amplifier system to generate a pulse width modulation (PWM) signal having a different switching frequency in each channel, and a signal processing method thereof.
2. Description of the Related Art
Generally, a digital amplifier pulse-width modulates a digital signal with a smaller amplitude, such as a pulse code modulation (PCM) signal, to generate a PWM signal. The digital amplifier amplifies the PWM signal to have a large amplitude by using a semiconductor switching device, such as a field effect transistor (FET). Then, an audio signal is extracted from the amplified PWM signal by using a low pass filter.
This digital amplifier may use only a single channel for mono sound, but in most cases, uses multi channels with two or more channels, such as 5.1 channels or 7.1 channels.
A conventional multi-channel digital amplifier will now be explained briefly. FIG. 1 illustrates waveforms of a conventional pulse width modulation (PWM) signal of the digital amplifier system. FIG. 2 illustrates waveforms of a conventional power-amplified PWM signal. First, a number of PWM signals (i.e., the number corresponds to a number of channels) are output through a PWM modulator. These output PWM signals are converted into N high output power PWM signals by N switching devices, respectively. At this time, the PWM modulator generates the PWM signals in respective channels by applying identical switching frequencies to all channels regardless of the number of channels, as illustrated in the waveform diagram of FIG. 1. That is, referring to FIG. 1, the PWM switching frequency of channel A and the PWM switching frequency of channel B are identical. In this case, the switching frequency can be determined by rising edges of the PWM Ch A and the PWM Ch B signals. As can be seen from FIG. 1, the rising edges of the PWM Ch A and PWM Ch B signals occur at the same point in time.
Referring to PWM Ch A and PWM Ch B of FIG. 1, although the channels are different, the identical PWM switching frequencies cause switching time points in the switching devices to occur at a high rate. However, if power is turned on or off at the same time at two or more channels, a power source unit experiences an overload such that power source noise is generated.
Additionally, as illustrated in FIG. 2, in the actual waveforms of the PWM signals amplified in the switching devices, a propagation delay time (td1) exists at a time when the switching device is turned on or off. However, as illustrated in FIG. 2, if the switching device of channel B is turned on at a time close to the time when the switching device of channel A is turned on, the propagation delay time (td1) of the switching device increases or decreases abnormally such that the waveform of the amplified PWM signal finally output is distorted.